Why did I bleed shortly after having ?

To start off I have been on birth control for about 6 months. Me and my boyfriend are ually active. I haven't missed a pill...Last week I took my placebo pills but only had some brown stuff for a couple days(my doctor said this is normal). I was still only taking the placebo pills but my period had gone away so me & my boyfriend had unprotected on Friday...I started a new pack that Sunday. Me and my boyfriend had today(Tuesday) & right after we finished after having (unprotected) I bleed only for about 15 minutes...Can someone help me to what this is?...I had/have a weird feeling in my stomach. I wouldn't call it a pain....Do i have an abrasion inside of me? Am I pregnant?....Also I heard this is what happens when a natural abortion occurs...Please help